Field:
The invention relates to the measurement of transiently evoked otoacoustic emissions (TEOAE), acoustic signals generated in the cochlea. In particular, it relates to the automated detection of contralateral suppression of TEOAE.
State of the Art:
Otoacoustic emissions (OAEs) are sounds actively generated in the inner ear. There are two types of otoacoustic emissions: spontaneous otoacoustic emissions (SOAEs), which can occur without external stimulation, and evoked otoacoustic emissions (EOAEs), which require an evoking stimulus. OAE can be measured with a small probe inserted into the ear canal.
People with normal hearing produce emissions. Those with hearing loss produce less or no OAE. Ears with cochlear hearing losses of more than 50 dB will normally not produce OAE.
EOAEs are currently evoked using three different methodologies: Stimulus Frequency OAEs (SFOAEs), Transient-evoked OAEs (TEOAE), and Distortion product OAEs (DPOAEs). SFOAEs are measured during the application of a pure-tone stimulus, and are detected by the vectorial difference between the stimulus waveform and the recorded waveform (which consists of the sum of the stimulus and the OAE). TEOAE are evoked using a click (broad frequency range) or toneburst (brief duration pure tone) stimulus. The evoked response from a click covers a frequency range that depends on the frequency range of the stimulus, typically up to around 4 kHz. A toneburst will elicit a response from the cochlear region that has the same frequency as the pure tone. DPOAEs are evoked using a pair of primary tones f1 and f2 with particular sound pressure level (usually either 65-55 dBSPL or 65 for both) and ratio (f1:f2). The evoked responses from these stimuli occur at frequencies (fdp) mathematically related to the primary frequencies, with the two most prominent being fdp=2f1−f2 (the “cubic” distortion tone, most commonly used for hearing screening) and fdp=f2−f1 (the “quadratic” distortion tone, or simple difference tone).
OAE are usually recorded with an averaging scheme, which takes advantage of the fact that the OAE signal is phase locked to the stimulus, while external noise is not. This results in the OAE signal being added in the averaging process, while noise is reduced.
For TEOAE, a so-called nonlinear stimulus sequence is often used to reduce stimulus artifacts in the recording. This makes use of the fact that TEOAE amplitude is not proportional to the stimulus amplitude. The stimulus is typically constructed from 4 single clicks, one of which is 3 times bigger in amplitude than the others. If the responses to the 3 lower amplitude clicks are subtracted from the response to the high-amplitude click, all linear components of the response, including stimulus artifacts, are cancelled out. One presentation and recording of such a sequence will hereafter be referred to as a frame.
There is an effect called contralateral suppression in OAF, which means that the OAE that can be recorded at one ear (“ipsilateral”) is influenced by acoustic stimulation of the other ear (contralateral). The contralateral acoustic stimulation is usually referred to as “CAS”.
OAE CAS suppression is thought to be caused by the efferent auditory system. The general concept of TEOAE CAS suppression is shown in FIG. 1.
This suppression effect will normally reduce the OAF level in the order of less than 1 dB, and can be seen in both TEOAE and DPOAE
In the case of DPOAE, CAS can produce suppression and also enhancement, depending on the presence of DPOAE fine structure at the frequency under test. DPOAE CAS suppression effects can be as high as 10 dB for single frequencies. However, since fine structure differs strongly between subjects, suitable frequencies have to be determined in a pre-measurement before the actual DPOAE suppression test is actually done. This makes the procedure inconvenient for a fast, automated test.
OAE are generated by the so-called outer hair cells in the cochlea, which act as an amplifier for soft sounds. Outer hair cells are connected to efferent nerve fibers, which can influence their motility and thus the generation of OAE.
The presence of OAE CAS suppression therefore does not only indicate cochlea functionality, but also the function of the efferent system on the ipsilateral ear and the afferent system on the contralateral ear
The auditory brainstem response (ABR) test gives information about the inner ear (cochlea) and brain pathways for hearing. This test is more generally referred to as auditory evoked potential (AEP). The ABR is also indicated for a person with signs, symptoms, or complaints suggesting a type of hearing loss on the auditory pathway.
The ABR is performed by pasting electrodes onto the head—similar to electrodes placed around the heart when an electrocardiogram is run—and recording electrical activity in response to sound. The person being tested preferably rests quietly or sleeps while the test is performed.
One normally needs to record ABR to check for retrocochlear disorders, which includes placing electrodes on the scalp and using electro-acoustic transducers for acoustic stimulation. In contrast, OAE CAS recording only needs the placement of two OAE probes.
CAS suppression in TEOAE in general is known, but not widely used mainly because the level difference between suppressed and unsuppressed recording of TEOAE is less than 1 dB and is hard to detect reliably; especially if noise, changes in sound probe placement or static pressure in the middle ear are present during testing.
To reduce artificial differences between the suppressed and unsuppressed measurement, tests are usually repeated, such as suppressed—unsuppressed—suppressed—unsuppressed. A constant drift of probe placement could then be separated from the suppression effect.
The difficulty in detecting this small level difference requires long test times, because detection of small level differences requires a high level accuracy, and is correlated to averaging time. Repetitions as described above further extend overall test time.
There thus remains a need for a fast, reliable, automated OAE CAS suppression test. The method and apparatus described below provides such a test.